Power cylinder assemblies of internal combustion engines generally comprise reciprocating pistons disposed within a cylindrical cavity, or cylinder, that has one closed end and one open end. The upper end or crown of the piston, together with the closed end of the cylinder, define a combustion chamber, where the fuel combustion takes place.
Many pistons for use with internal combustion engines have three piston rings, each located in an annular groove defined in the end of the wall of the piston near the crown. The two piston rings closest to the crown are known as compression rings. The uppermost compression ring seals the combustion chamber from leakage during combustion of the air-fuel mixture, and transfers heat from the combustion chamber to the cylinder wall. Of the three piston rings, the upper compression ring operates in the harshest environment in terms of pressure, temperature and minimal lubrication.
The upper compression ring in a piston is generally a split ring. It is compressed to fit within the cylinder. When the engine is operating, the pistons and piston rings are subjected to extremely harsh conditions, including high pressures and temperatures, and repeated rapid reciprocal movements. As the piston moves within the cylinder the piston rings are in contact with, and move up and down against, the cylinder walls. The reciprocating motion of the pistons and the piston rings within the cylinders, along with the high operating pressures and temperatures, can subject the piston rings to wear and possible failure. Since piston rings are difficult to access when the piston is in place in an internal combustion engine, replacing a piston ring may be very expensive and time consuming. Therefore, it is important that the piston rings be durable and resistant to scuffing and wear.
To increase the durability of piston rings and to enable piston rings to withstand the harsh conditions to which they are subjected, very hard materials may be used to coat the outer perimeter of piston rings. Compression rings, which may see the highest temperatures and pressures, are often coated with chromium nitride, or a similar hard coating to increase durability. This hard, outer coating may be applied by physical vapor deposition (PVD), or by other known mechanisms.
The hard coating on the piston ring is deposited directly on the relatively soft substrate material of the piston ring, thereby creating a steep transition in hardness between the layers. This may lead to an increased risk of cracking in the hard outer layer of the piston ring. Furthermore, the coating material may be brittle and therefore prone to cracking, especially at the outer corners of the ring. Once cracks start to form in a piston ring, they may propagate, and can ultimately lead to breakage and failure of the piston ring
A ductile surface at the outer corners of a piston ring may increase the durability of the rings. However, it is also important to retain the hard surface on the remainder of the outer circumference of the piston ring to reduce wear.